Rapid prototyping processes are well known, and employ known layered manufacturing techniques wherein an article (e.g. metal casting mold, prototype part, etc.) is progressively made in a series of layers sequentially built-up one upon the next. One variant of such process is the so-called SLA 1 System wherein a computer-controlled, focused UV laser is scanned over the top surface of a bath of a photopolymerizable liquid polymer to selectively polymerize the polymer where the laser beam strikes it, and thereby form a first solid polymeric layer at the top surface of the bath. This solid layer is then lowered into the bath such that a fresh layer of liquid polymer covers the solid layer. The laser polymerization process is then repeated to generate a second solid polymeric layer, and so on, until a plurality of superimposed solid polymeric layers complete the desired article. Another variant of the rapid prototyping process is known as the Selective Laser Sintering (SLS) process, wherein a computer-controlled laser beam sinters selected areas of multiple layers of loosely compacted powder (e.g. plastic, metal, ceramic, wax etc.), layer-by-layer, until the article is completely built-up. The SLS variant is described in more detail in U.S. patent Bourell et al. U.S. Pat. No. 5,076,869 issued Dec. 31, 1991. Still another variant is known as the “3D-Printing” rapid prototyping process wherein a computer-controlled ink jet printing device (e.g. continuous stream, or drop-on-demand type) propels a stream of binder from one or more jets onto select areas of a first layer of loose particles (i.e. about 60 μm to about 140 mμ in diameter) according to a pattern dictated by the computer. A microcomputer, such as is used in computer assisted design (CAD) work, controls the jets according to a three dimensional (3D) computer model of the article, and two dimensional (2D) slices taken therethrough, as is well known to those skilled in the art. Thereafter, a second layer of loose particles is laid atop the first layer, and as with the first layer, the ink jet selectively directs a stream of binder onto the second layer of particles. The particle layers may be formed by depositing either dry particles, or particles suspended in a volatile liquid, onto a working surface before the binder is applied. When used, the volatile liquid is allowed to evaporate, at least partially, from a first layer before depositing a second layer thereon. This process is repeated, over and over, layer after layer, until the article is completed. The binder in one layer is at least partially hardened (e.g. dried or cured) before the next layer of particles is laid down. The finished article may thereafter be heated for further drying/curing of the binder to provide the article with sufficient green strength for handling, and to permit separating of the loose, unbonded particles from the bonded particles without distorting or damaging the article. Thereafter, the article may further be heated to sinter or weld the particles together to form a finished, albeit porous, article. Depending on the intended use of the article, it may or may not thereafter be infiltrated with a suitable infiltrant (e.g. a polymer, or a metal having a liquidus temperature lower than that of the sintered metal particles) to seal the article and eliminate the porosity. The 3D-Printing rapid prototyping process is described in more detail in U.S. patents Sachs et al U.S. Pat. No. 5,204,055 (issued Apr. 20, 1993), Cima et al. U.S. Pat. No. 5,387,380 (issued Feb. 7, 1995), and Sachs U.S. Pat. No. 6,036,777 (issued Mar. 14, 2000) which are herein incorporated by reference.
It would be desirable to manufacture light-weight articles from particles made from aluminum or magnesium or their alloys (hereafter Al/Mg particles) using the 3D-Printing rapid prototyping technique. However, it has not heretofore been possible to do so owing to the reactivity of Al/Mg particles and their propensity to readily oxidize in air to form an oxide skin on the particle's surface that impedes sintering/welding of the particles to each other.